No hacks are allowed. However legit mods, that don't give you an unfair advantage are allowed. This is a short list of mods that are generally allowed, but keep in mind the rules are always in effect, and that a mod being listed here does not guarantee that it is not possible to misuse it. This list is far from exhaustive, you can use your own judgment as to whether a particular mod violates the rules, or if in doubt ask an admin.
List of mods that are allowed
- Custom texture packs that don't let you see through dirt, stone, sand, gravel, cobblestone or other opaque blocks
- Singleplayer mods (as long as they aren't useable in SMP without OP) mods such as singleplayercommands, and toomanyitems
- Minimap mods, as long as you disable players and caves or other underground things from being visible
- Improved chat